A light emitting diode (LED) is a semiconductor device capable of converting electrical energy into visible light and radiation energy when electrical current flows between the anode and the cathode due to a voltage applied on both terminals of the semiconductor device. When the current passes through the LED in the forward direction, electrons recombine with holes and the extra energy is released in the form of light. The wavelength of the emitted light corresponds to the material and the energy associated with electron-hole pair recombination. The advantages of using the LED include a low operating voltage, low power consumption, high illuminating efficiency, very short response time, pure light color, high structural firmness, high impact resistance, excellent performance reliability, light weight, cost effectiveness, long service life, and so on. Therefore, the incandescent bulbs or mercury vapor lamps used in the conventional lighting system are gradually replaced by LEDs in many applications.
By using three primary color LEDs, for example a combination of red (R), green (G) and blue (B) LEDs, and adjusting the brightness of the LEDs, output light beams with various emission colors can be produced. Generally, the lighting system has a user operation interface (e.g. a button or a knob) or a remote controller. By triggering the user operation interface or using the remote controller, the brightness or the color of the output light from the lighting system can be controlled accordingly. Recently, an ultrasonic transceiver has been employed in the lighting system so as to adjust the light strength or the light color.
FIG. 1A is a schematic diagram illustrating a lighting system with an ultrasonic transceiver to control the light strength or the light color according to the prior art. As shown in FIG. 1A, the lighting system comprises a light source 10 and an ultrasonic transceiver 11. The light source 10 comprises a red (R) LED, a green (G) and a blue (B) LED. When an object 12 (e.g. a user's hand) enters the sensing range of the ultrasonic transceiver 11, an ultrasonic signal emitted by the ultrasonic transceiver 11 is reflected by the object 12, and the reflected ultrasonic signal (or an echo signal) is then transmitted to a receiver of the ultrasonic transceiver 11. Upon receipt of the echo signal, the processor of the lighting system may measure the time of flight (TOF) of the ultrasonic signal. In the context, the time of the ultrasonic signal emitted from the ultrasonic sensor and reflected by the object to reach the receiver of the ultrasonic sensor is referred as the time of flight (TOF). According to the TOF, the distance R between the object 12 and the receiver of the ultrasonic transceiver 11 can be deduced. According to a change of the distance R, a control signal is generated. In response to the control signal, the light source 10 of the lighting system can produce light with adjustable optical characteristics including the light strength or the light color.
FIG. 1B is a schematic diagram illustrating another lighting system disclosed in WO 2006/056814. As shown in FIG. 1B, the lighting system principally comprises an infrared transceiver 13 and a light-emitting unit 13. When an object 12 (e.g. a user's hand) enters the sensing range of the infrared transceiver 13, an infrared beam 15 emitted by the infrared transceiver 13 is reflected by the object 12, and the reflected infrared beam 16 is then transmitted to an infrared receiver 17 of the infrared transceiver 13. Generally, the intensity of infrared light 16 reflected from the object 12 and received by the infrared transceiver 13 is dependent on the inverse square of the distance between the infrared transceiver 13 and the object 12. By determining the movement of the object 12 away from or toward the infrared transceiver 13, the brightness or the color of the output light from the light-emitting unit 14 of the lighting system is adjustable accordingly.
The above lighting systems, however, still have some drawbacks. For example, only one of the optical characteristics can be adjusted at a time. Since the light strength adjusting operation and the light color adjusting operation fail to be simultaneously done, the conventional light-adjusting methods are not user-friendly. Therefore, there is a need of providing an improved light-adjusting method to obviate the drawbacks encountered from the prior art.